Magnetic deflecting means for cathode-ray tubes



July 15, 1969 MAST 3,456,149

MAGNETIC DEFLECTING MEANS FOR CAIHOUE-RAY TUBES Filed Feb. 9, 1965 0 INVENTORQ JOHANNES B. 6. S. VAN MAST AGE States ate ABSTRACT OF THE DISCLOSURE Means for reducing the residual magnetism in the ferromagnetic yoke of a CRT deflection system comprises a two-terminal network including, in series circuit, a compensating coil and an adjustable resistor. The network is directly connected in parallel with the deflection coil and has a time constant that is higher than the time constant of the coil whereby a reverse current is caused to flow through the deflection coil during the retrace period of the sawtooth current.

The present invention relates to magnetic deflecting means for a cathode-ray tube and, more particularly, to means for eliminating the residual magnetism in a deflection coil system incorporating a ferromagnetic yoke.

The application of a ferromagnetic yoke in a deflection coil system is known to be favourable because it considerably improves the deflection eflicicncy. In addition, for a given strength of the deflection field, it allows a substantial reduction of the number of ampere-turns in each of the deflection coils. A disadvantage is, however, that during the retrace period of the scanning beam, when the deflection current in the coils is reduced to zero, the deflection field does not entirely collapse owing to the residual magnetism in the ferromagnetic yoke. In order to overcome this disadvantage, it has been proposed to change the damping of the deflection coils from a critical Value during the deflection period of the scanning beam to an undercritical value during the retrace period of the scanning beam. This causes the electromagnetic energy that is stored in the coil after each deflection of the scanning beam to decay towards zero through a number of decaying oscillations, thereby destroying the residual magnetism in the yoke. An important advantage of this known method is that it will also yield this desired effect if the peak-values of the deflection currents vary irregularly, which is the case, for instance, with the deflection currents supplied to the deflection coils of a cathode-ray tube that is used for the P.P.I. display of radar echos and interscan markers. A severe drawback of this known method is, however, that it requires relatively complicated switching and control means, and in addition its use is limited by the fact that the relatively long decay time is not always available.

One object of the invention is to provide improved means for eliminating the residual magnetism in the yoke of a deflection coil system so as to overcome the above mentioned limitations of the known method while retaining the advantages of a ferromagnetic yoke. Another object of the invention is to provide an improved means for eliminating the residual magnetism comprising simple adjusting means that allows for ease in reproducing the desired effect.

According to the invention, means are provided for eliminating the residual magnetism of the yoke comprising a plurality of separate two-terminal networks corresponding in number to the number of deflection coils.

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Each one of the two terminal networks comprises an adjustable resistor and a compensating coil. Each one of said two-terminal networks is directly connected with a diflerent one of said deflection coils so that each two-terminal network is arranged in shunt-relationship with its associated deflection coil exclusively by way of the direct connecting means so as to wholly or partially bridge its associated deflection coil.

In order that the invention may be clearly understood and readily carried into elfect, it will be described in greater detail with reference to the accompanying drawings, in which FIG. 1 represents diagrammatically and partially in block diagram form an embodiment of the deflection coil system according to the invention, in which only one pair of deflection coils is shown; and FIG. 2 shows a set of wave forms indicating various signals which occur in the circuit of FIG. 1 during its operation.

Referring to FIG. 1, there is shown a magnetic deflection circuit for a cathode-ray tube 1. In such a circuit it is common practice to provide the cathode-ray tube with two pairs of orthogonal deflection coils. For the sake of simplicity, only one pair of deflection coils L resp. L is shown in FIG. 1. The deflection coils are provided with an annular yoke 2 of ferromagnetic material which is schematically represented. These deflection coils are symmetrically energized by the sawtooth current generator 3, and are critically damped by means of the usual damping resistors R resp. R The circuit is further provided with means 4 for eliminating the residual magnetism of the yoke.

According to the invention, the latter means comprise a plurality of separate two-terminal networks corresponding in number to the number of deflection coils. Each one comprises an adjustable resistor R resp. R and a compensating coil L resp. L Each one of said twoterminal networks, L R resp. L R' is associated with a different one of said deflection coils L resp. L and each two-terminal network is connected in shunt relationship with its associated deflection coil, exclusively by way of direct connecting means 5, 6, 7 so as to wholly or partially bridge its associated deflection coil.

The manner in which the means for eliminating the residual magnetism operate will now be explained with reference to FIG. 2, wherein three consecutive time-intervals may be distinguished:

(1) the time interval t which extends from the moment T (start of the saw-tooth) to the moment T (end of the saw-tooth);

(2) the time-interval t which extends from the moment T (end of the saw-tooth) to the moment T (the time position of this latter moment will be elucidated further below);

(3) the time-interval t which extends from the moment T to the moment T (start of the new saw-tooth).

During the time-interval t a sawtooth generator 3 provides a current i which splits up into:

( 1) a current i through the deflection coil L (2) a current 1 through the damping resistor R (3) a current 1 through the compensating coil L and the resistor R Owing to the aforementioned direct connecting means 5, 6 and 7, a fixed relation exists between the current 1' and the current i said relation being independent of the peak value of the current i The direction of the currents i and i;,, is indicated in FIG. 1 by means of arrows. At the end of the time interval t the current i of the saw-tooth generator 3 suddenly drops to zero. The currents in the coils L and L however, do not follow this sudden drop to zero, but will decrease at a much slower rate owing to the energy built up in the magnetic field. Accordingly, the current in the deflection coil L and the compensating coil L will at first continue to flow in the same direction as it did during the time-interval t As the output impedance of the saw-tooth generator is very high, these currents find their return path mainly through the damping resistor R Thus, as soon as the saw-tooth current i at the instant T drops to zero, the current through the said damping resistor will reverse its sign. The time constant determined by the deflection coil L and the damping resistor R and the time constant determined by the compensating coil L and the adjustable resistor R are suitably chosen so that the current trough the coil L decreases faster than the current through the coil L The current in the coil L will then, after some time, find its return path not only through the damping resistor R but also through the deflection coil L It will be evident that the direction of current flow in the coil L will now be opposite to the direction of current flow that occurs during the time interval t In FIG. 2 the instant at which the current through the coil L reverses its direction is represented by T It will also be clear, that by this reversal of direction of the current through the coil L the residual magnetism from the previous excitation cycle will be decreased. The resistor R now can be adjusted so that at the instant T at which the generator 3 starts a new saw-tooth current, the residual magnetism is just completely eliminated.

As the current through the coil L now starts at a negative value, the magnitude of which is adjusted so that the residual magnetism is eliminated, a new beam deflection will be independent of the previous one.

The above explanation can of course equally well be applied to the components which in FIG. 1 are indicated by accented reference indices.

The fact tht the compensating coil and the deflection coil are not inductively coupled allows the means according to the invention to be located somewhere remote from the cathode-ray tube. This feature is important because it prevents the desired compensating effect from being disturbed by leakage fields and eddy currents in metal parts which often exert influences that are hard to establish. The latter feature and the simplicity of the means for adjustment result in a particularly easy reproducibility of the desired compensating effect. Preferably the compensating coil and the adjustable resistor of a two-terminal network are connected in series. It is also possible to connect these components in parallel if such would be better suited, eg in case such a parallel arrangement would yield more appropriate dimensions.

The embodiment shown in FIG. 1 may also be modified in such a way that each deflection coil is only partially bridged by its associated two-terminal network.

Finally, it may be observed that in a practical embodiment of an arrangement according to the invention, in which the compensating coil and the adjustable resistor were connected in series, the following dimensions were successfully applied:

Deflection coil L mh 1.5 Damping resistor R 3909 Compensating coil L mh 200 Adjustable resistor R 2009 What I claim is:

1. Magnetic beam deflecting means for a cathode-ray tube comprising, a deflection coil system including a ferromagnetic yoke, and means for eliminating the residual magnetism in said yoke comprising, separate two-terminal networks corresponding in number to the number of deflection coils, each of said networks comprising a compensating coil and an adjustable resistor for adjusting the time constant of said network as a function of the residual magnetism level in said yoke, means directly connecting each one of said two-terminal networks with a different one of said deflection coils so that each two-ter- 4 minal network is arranged in'shunt relationship with its associated deflection coil exclusively by way of said direct connecting means so as to wholly or partially bridge its associated deflection coil.

2. Magnetic deflecting means as claimed in claim 1 wherein the adjustable resistor and the compensating coil are connected in series in each one of said two-terminal networks.

3. Magnetic deflecting means as claimed in claim 1 wherein the time constant of each one of said two-terminal networks is substantially higher than the time constant of its associated deflection coil.

4. Magnetic deflecting means as claimed in claim 2 wherein the time constant of each one of said two-terminal networks is substantially higher than the time constant of its associated deflection coil.

5. In combination, a ferromagnetic core, a coil wound on said core, a two-terminal network comprising a compensating coil and a resistor connected together, means directly connecting said two-terminal network in parallel with a portion of said coil, a source of periodic current of a given frequency, means for coupling said current source to said coil and network so that a current flows in said coil in a given direction, said two-terminal network having a time constant that is dependent upon the level of residual magnetism produced in the core by said current flow in the coil, and damping means coupled to said coil so as to provide a current path for dissipating the energy stored in said coil.

6. A combination as claimed in claim 5 wherein said compensating coil and said resistor are connected in series.

7. A combination as claimed in claim 6 wherein said current source comprises a sawtooth current generator that supplies a sawtooth current to said coil and to said two-terminal network, the L/R time constants of said twoterminal network and said coil being chosen so that, in the period between the end of one sweep period and the start of the next sweep period of the sawtooth current, said two-terminal network supplies a reverse current to said coil that substantially reduces any residual magnetism in the core produced by the sawtooth current flow in said coil.

8. A combination as claimed in claim 7 wherein said resistor is adjustable and the time constant of said twoterminal network is higher than the time constant of said coil, and said damping means comprises a resistor connected in parallel with said coil and said two-terminal network.

9. A combination as claimed in claim 8 wherein the resistance value of said damping resistor is chosen so that said coil is critically clamped at said given frequency of the sawtooth current.

10. A combination as claimed in claim 6 further comprising a second coil wound on a ferromagnetic core and coupled to said current source, a second two-terminal network comprising a second compensating coil and a second resistor connected in series, means directly connecting said second two-terminal network in parallel with a portion of said second coil, and means coupling said second coil to said damping means so as to providea current path for dissipating the energy stored in said second coil.

References Cited UNITED STATES PATENTS 2,114,841 4/1938 Hanna 3l7l57.5 2,228,821 1/1941 Hansen 31527 2,279,849 4/1942 Warrington 3l7l57.5 2,007,380 7/1935 Morlock 315-27 3,153,174 10/1964 Claypool et al. 31527 RICHARD A. FARLEY, Primary Examiner I. G. PAXTER, Assistant Examiner U.S. Cl. X.R. 307 101; 3l7l57.5 

